1. Field of the Invention
The present invention relates to a chattering eliminating apparatus.
2. Description of the Related Art
In an electric apparatus including a contact switch and a control apparatus receiving the output signal of the contact switch, when the contact switch is turned ON and OFF alternately for very short times, the control apparatus may erroneously operate due to a chattering phenomenon in the output signal of the contact switch.
In order to prevent the control apparatus from erroneously operating due to the chattering phenomenon, a chattering eliminating apparatus is inserted between the contact switch and the control apparatus.
A first prior art chattering eliminating apparatus (see: JP-A-2-162821) is constructed by a charging/discharging circuit for charging/discharging a capacitor in accordance with an input signal and a comparator for comparing the voltage of the capacitor with a reference voltage. This will be explained later in detail.
In the above-described first prior art chattering eliminating apparatus, however, it is impossible to completely eliminate the chattering phenomenon when the input signal has a duty ratio of much larger than 50% or much smaller than 50%. Also, in order to completely eliminate the chattering phenomenon, the capacitance of the capacitor has to be larger, which would increase the size of the apparatus.
A second prior art chattering apparatus (see: JP-A-5-83093) is constructed by an exclusive OR circuit for detecting a chattering phenomenon in an input signal, a monostable (one-shot) multivibrator triggered by a rising edge of the output signal of the exclusive OR circuit, a T-type flip-flop triggered by a training edge of the output signal of the one-shot multivibrator, and a D-type flip-flop for fetching the output signal of the T-type flip-flop in synchronization with a rising edge of the output signal of the one-shot multivibrator. The output signal of the T-type flip-flop is fed back to the exclusive OR circuit. This also will be explained later in detail.
In the above-described second prior art chattering eliminating apparatus, however, since the one-shot multivibrator is usually constructed by a differential circuit, the one-shot multivibrator, i.e., the chattering eliminating apparatus is subjected to noise.
A third prior art chattering eliminating apparatus (see: JP-A-4-274613) is constructed by an exclusive OR circuit for receiving an input signal and an output signal to generate a reset signal, a counter which is reset by the reset signal and is counted down by receiving a clock signal and an initial value, and a D-type flip-flop for generating the output signal. The D-type flip-flop fetches the output signal in synchronization with a carry signal generated for the counter. This also will be explained later in detail.
In the above-describe third prior art chattering eliminating apparatus, however, since there is no capacitor for absorbing noise, the chattering eliminating apparatus is subjected to noise.